Ceramic articles have many uses including catalyst supports, dental porcelain, heat exchangers, turbine blades, substrates for integrated circuits, etc. The particular ceramic which is used in a given application depends on the properties required for the given application. For example, leucite ceramics can be used as dental porcelains, coatings for metals and metal/ceramic seals. A review of the importance of potassium aluminosilicate compositions in dental ceramics is given in C. Hahn and K. Teuchert in Ber. Dt. Keram. Ges., 57, (1980) Nos. 9-10, 208-215. One drawback to the use of leucite in dental applications is that it is fragile and hard to repair. For this reason, dental restorations usually require a metal framework. Accordingly, there is a need for a leucite ceramic with higher strength. There is also a need for a process which can form a leucite ceramic at lower temperatures so that the processes of high temperature glass melting followed by fritting and milling are eliminated.
U.S. Pat. No. 4,798,536 teaches the addition of potassium salts to various feldspars to produce a porcelain having a greater amount of a leucite phase and increased strength. Applicants have produced a partially crystallized leucite glass ceramic, with strengths greater than those reported in the '536 reference, by taking a potassium exchanged zeolite Y powder and heating it at a temperature of about 1050.degree. C. to give an amorphous powder. This amorphous powder is then formed into a desired shape and sintered at a temperature of about 1150.degree.-1400.degree. C. to give a leucite ceramic article. Thus, glass melting and preparation of frits are unnecessary.
Although the prior art describes the preparation of ceramics from zeolites, there is no report of a process to make a dense leucite ceramic article. For example, D. W. Breck in ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York (1974), pp. 493-496 states that Mg-X can be heated to form cordierite. The disclosed process involves heating the Mg-X zeolite at 1500.degree. C. to form a glass and then heating the glass above 1000.degree. C. to form cordierite. Thus, two steps are required to form cordierite.
Another reference which teaches the preparation of a cordierite based ceramic article is U.S. Pat. No. 4,814,303 to Chowdry et al. Chowdry discloses producing a monolithic anorthite, anorthite-cordierite or cordierite based ceramic article by heating the Ca, Ca/Mg and Mg forms of zeolites X, Y and A at a temperature of about 900.degree. C. to about 1350.degree. C. Example 33 of Chowdry discloses preparing a potassium exchanged zeolite X followed by sintering at 1000.degree. C., thereby forming predominantly KAlSi.sub.2 O.sub.6 which supposedly showed the X-ray diffraction pattern of leucite (JCPDS File No. 15-47).
Finally, European Patent Publication Number 298,701 (to Taga et al.) describes the preparation of a ceramic article having an anorthite phase from a calcium zeolite. The process involves a calcination to form an amorphous product which can then be shaped into an article and sintered at temperatures of about 850.degree.-950.degree. C.
Applicants' process differs considerably from this prior art. First, the instant process is a two-step process whereas Chowdry discloses a one-step process. As the examples herein show, a two step process is critical for producing usable ceramic articles. Second, the type of zeolites used and sintering conditions used in the instant process are completely different from that in the Taga reference.
The process of this invention can also be used to produce ceramic articles whose principal crystalline phase is pollucite. Pollucite ceramic articles can be used in applications where there is a need for low thermal shock and high refractoriness since pollucite has a coefficient of thermal expansion of less than 2.times.10.sup.-6 .degree.C..sup.-1 over the temperature range 50.degree.-700.degree. C., and has a melting point of greater than 1900.degree. C. This type of ceramic article can be produced by using a cesium exchanged zeolite instead of a potassium exchanged zeolite and sintering at a temperature of about 1250.degree. C.
Another drawback to leucite in certain applications is it has a large coefficient of thermal expansion. Leucite goes through a phase change (from tetragonal to cubic) at a temperature between 400.degree. and 600.degree. C. which results in a unit cell volume increase of about 5%. Even at temperatures below this structural transition, leucite and its glass ceramics show relatively large thermal expansion coefficients. The prior art describes that thermal expansion in leucite glass ceramics can be varied over a somewhat narrow range by changing the ratio of leucite crystals to residual glass in the sintered ceramic. This method of thermal expansion variation is described in U.S. Pat. No. 4,604,366, which teaches that thermal expansion can be adjusted over a range of 10.times.10.sup.-6 to 19.times.10.sup.-6 by blending two different glass frits with two different pulverized glass ceramic powders in varying ratios.
Applicants have also discovered a process by which the coefficient of thermal expansion of the leucite can be varied from about 2.times.10.sup.-6 to about 27.times.10.sup.-6 .degree.C..sup.-1 in the 50.degree. to 700.degree. C. temperature range.
The coefficient can be varied by introducing a pollucite phase into the leucite ceramic. Pollucite is a relatively low thermal expansion cesium-silica-alumina ceramic which has the cubic high-leucite structure at room temperature and forms a continuous series of solid solutions with leucite over the full subsolidus temperature range. As the cesium level in the leucite ceramic is increased the thermal expansion coefficient decreases to a point that the leucite/pollucite assumes the high leucite cubic structure at room temperature, after which time the coefficient of expansion continues to decrease with increased cesium content.
The leucite/pollucite ceramic article can be made by exchanging a zeolite such as zeolite Y with both potassium and cesium and then following the process described above. By varying the amounts of potassium and cesium content in the starting zeolite and processing as described above, one can obtain any desired leucite/pollucite solid solution. The use of a potassium and cesium exchanged zeolite as the starting material provides a uniform distribution of these cations in the starting zeolite which in turn results in a homogeneous distribution of these cations in the ceramic article. By varying the amounts of cesium and potassium in the starting zeolite, the thermal expansion coefficient of the ceramic article can be "tuned" to whatever value is desired between the coefficients given above. Thus, the instant process greatly simplifies the control of the coefficient of thermal expansion over that found in the prior art and allows a wider range of the thermal expansion coefficient to be attained.